1st attempt at increasing performance on a 2001 GTX carb version...

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Have a good engine with not many hours since rebuild. Compression showed 130 psi on both cylinders. Would run about 6800 – 6900 rpm WOT.

  • Solas Concord ST-CD-15/20 Impeller
  • A 2 or 3 degree wedge (can't remember for sure) between pump and steering nozzle.
  • Carbon Tech Reeds (206-SS)
  • ProK Flame Arrestors
  • Re-jetted carbs to 85 lows, 167.5 highs
  • 2.3 Needles and seats
  • Accelerator pump is still operating
  • AD Racing billet head with 155 PSI domes.
  • LS screws at 1 ½ out
  • HS screws at ¾ out
  • Pop off at 15 psi
Engine starts easily, idles smoothly. If I smoothly increase throttle from idle, engine stutters until rpms reach 4,000. After that, smooth as silk. If I nail the throttle from idle, engine stutters to about 5,000 then accelerates quickly. Still, will only top out at about 6800+ rpm.

Also sounds different coming off idle.... Sounds more "metallic." I am assuming this is because of the absence of the airbox, maybe? Noise disappears just above idle speed.

Would run 57 on GPS before. My goal was to break 60. Still tops out about 57.

What do I need to be adjusting, checking, etc.

Thanks for any assistance.
What you did (approximately) I did to a 2000 XP and if FLEW! GPS at 64. I ran the engine for more than a decade with no issues. You MUST be very careful about getting water into the engine when using the ProK flame arrestors.
Can you do a video on the noise?
What do the plugs look like?
On the stuttering issue, could it be running too rich?
It’s better to error on being too rich than lean. I would go richer on the lows and see if it gets worse then go leaner from there.
Boyd12... I have the covers on the Proks. Hopefully that will help. The way I ride, I've never had an issue with water getting in the hull.

Will do a video on the noise when possible.

Will post the plugs when possible.

I suspected that the stuttering is because it's rich. Just wanted a second opinion. Turned the LS screws in from 1 1/2 to 1 1/4. Little if any difference. I may need to up the pop off to 17 or 18. Afraid to go much lower on the LS screws. Any idea what absolute minimum is on the LS screws?

Mikidymac.... Understood. Will try both ways
The covers will help, but you still need to keep an eye on water in the hull as a mistake can be VERY costly!
Mikidymac is correct. It is better to run rich.
as for the LS screws, check the plugs and see what they look like. if they are too dark, you can start leaning it out a little at a time. Do you have the Mikuni carb manual? If not, let me know. I think I have a scanned copy somewhere.
Update... Haven't had an opportunity to look at the plugs, but I may have figured out what the "metallic" noise is. Was looking for something in my shop and found a GTX driveshaft that is perfectly straight. This means that an older, less than straight driveshaft is probably in my Doo making a lot of racket. I'll let you know what I find out.

PS... Have been studying the Mikuni manual.
Replaced the driveshaft with the straight one. Reduced but did not eliminate the noise. Sad thing is, I can't remember if it's always had this noise or not.

I turned the LS screws in to 1 1/8 turns. Turned the HS screws out to 1/2 turn. Nothing seemed to change on low end. Too rough to see about top end. Got run out of the water by a storm so still haven't looked at plugs.
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